Electron source



June 2, 1953 J, 00 2,640,949

ELECTRON SOURCE Filed Feb. '7, 1951 ACCELERA r/Ne HEATER VOLTAGE CURRENTSUPPLY SUPPLY IN V EN TOR. L551. IE J COOK Patented June 2, 1953ELECTRON SOURCE Leslie J. Cook, Berkeley, Calif., assignor to the UnitedStates of America as represented by the United States Atomic EnergyCommission Application February 7, 1951, Serial No. 209,768

6 Claims. 1

The present invention relates to an improvement in electron sources andis in particular concerned with an improved method and means forproducing copious quantities of electrons in a controlled direction witha minimum power expenditure.

Of the known electron sources, probably the most efficient for highelectron emissivity is the diffusion type wherein a material, such asthorium oxide, is diffused through a porous metal cathode structure andforms a thin electron emissive layer on the surface thereof which iscontinually replaced by the diffusion of more material through the metalof the cathode structure. An early advance in this art was made by A. W.Hull et al., Patent No. 2,107,945, who disclosed a woven or meshstructure enclosing an oxide of alkaline earth and a heater wherebydiffusion through the mesh of the enclosed material creates an electronemissive surface exterior thereof. Numerous modifications andimprovements upon this structure have been since advanced by others inthe field; however, all require a relatively large power expenditure forany particular electron beam developed thereby. This requirementnecessarily imposes limitations upon minimum size and associated powersupplies which are generally undesirable and, at least in certainapplications, are particularly troublesome. The present invention, byovercoming these requirements or limitations, provides an electronsource particularly adaptable for electron beam apparatus, such as forexample, certain types of vacuum tubes, cathode ray tubes, ion sources,and electron accelerating equipment; and this is achieved by theaccomplishment of the following objects.

It is an object of the present invention to provide an improved electronsource.

It is another object of the present invention to provide an improvedmethod and means for producing an intense electron beam.

It is another object of the present invention to provide a method andmeans for producing an improved heat controlled electron emissivesurface.

It is a further object of the present invention to provide an improvedelectron source having small power dissipation.

Numerous other objects and advantages of the invention will be apparentfrom the following disclosure taken together with the attached drawingswherein;

Figure 1 is a perspective view of a preferred embodiment of an electronsource constructed in accordance with the principles of the invention;

Fig. 2 is a sectional view of the embodiment of Fig. 1 taken on plane2-2 of Fig. 1;

Fig. 3 is a sectional view of the embodiment of Fig. 1 taken on plane 33of Fig. 2; and

Fig. 4 is a schematic illustration of one application of the improvedelectron source of the present invention and including cooperating powersupplies.

Considering a preferred embodiment of the invention as shown in Figs. 1,2, and 3 it will be seen that the electron source I comprises a firstopen-faced box 2 of generally rectangular configuration and having apair of slots 3 therein extending away from one edge of the open face ofbox 2. Also provided is a second box or source face 4 consisting of agenerally rectangular boxshaped element having an open face and havingoutside dimensions substantially identical to the inside dimensions ofbox 2 whereby face 4 slip fits into box 2 to form a closed cathodeenvelope 6. Source face 4 is preferably formed of a metal as noted belowand is also provided with a pair of slots 1 extending away from an edgeof the open side of source face 4 in a position to align with slots 3 inbox 2 when face 4 is assembled with box 2. It will be appreciated thatthe slipfit between face 4 and box 2 may be accomplished by making face4 large enough to slip over the sides of box 2 or alternatively thatinnumerable types of mechanical connections may be made between face 4and box 2 to form a desired envelope structure 6.

Envelope 6 is preferably supported by conductors 8 which may consist ofrods or legs as shown and which extend into envelope 6 and thereby servethe dual function of supporting envelope 6 and providing electricalconnections internal thereto. Envelope 6 is mounted upon conductors 8 bymeans of the slots 3 and l in box 2 and source face 4, respectively, andin assembly conductors 8 are slipped into slots 3 in box 2 and face 4 isthereupon slipped into contact with box 2 with slots 1 thereincooperating with conductors 8. The depth of slots 3 and 1 thus determinethe degree of engagement of face 4 and box 2 so that the depth of theseslots is designed to produce the desired relation between face 4 and box2. In order to prevent electrical contact between envelope 6 andconductors 8, insulators 9 are provided about conductors 8 at the pointwhere they engage the edges of slots 3 and 1 in box 2 and face 4, andthe inner ends of these slots may be formed in such a configuration thatthey make intimate contact with insulators 9 on conductors 8; therebysubstantially sealing the interior of envelope 5 at the point of entryof conductors 8.

Centrally disposed within envelope 6 is a heater unit H which extendsbetween the ends of conductors 8 and is energized therethrough. Also itis only necessary for the activating material I 2 to extend between theheater and the inner surface of source face t with various mechanicalschemes being adaptable to achieve this disposition.

An electron emissive surface is provided' on electron source 9 by theinclusion of one or more minute apertures It in face a which in theillustrated embodiment are circular and alignedalong the center of facel. Apertures 53 provide for the flow of vapor from activating material i2 interior to envelope to the outer surface of face t where an electronemissive surface is thereby formed.

Activating material i2 may consist of any suitable material, such as anoxide of an alkaline earth, which is volatile and which in combinationwith the material of face d forms an electron emissive surface. Also,face d is formed of a material which satisfactorily cooperates with theactivating material to form an electron emissivc surface having'thedesired properties. An exampleof a suitable combination is an activatingmaterial of barium aluminate and a face l formed of zirconium. The workfunction of the electron emissive surface formed depends upon thematerials cooperating to form. the surface and. by proper choice ofmaterials a very low work function may be obtained.

In operation the heater unit H is energized through conductors 3 as by acurrent source con.- nected therebetween. The heat produced by heaterunit ll causes a part of the activating material. I2 to volatilize andpass through apertures I3 in source face l. Thisvapor passing throughapertures it extends about theapertures to form a layer of substantiallymonoatoinic thickness upon the outer surface of face 4 directly aboutapertures It. The heat from energized heater unit II also raises thetemperature of face 4 to an extent that electrons are emitted from thesurface directly about apertures l3 where the monoatomic layer ofactivating mate.- rialis disposed. Electrons are thusemitted from only avery limited area of face t as determined by the disposition ofapertures 43 therein. In practice, apertures is are very minute and infact the entir source I i generally suite small, of the order ofone-half inch long, and a virtual line source may be obtained bypositioning apertures is close enough together that the electronemissive surface about each abuts the electron emissive surface of thenext adjacent. Although it is apparent that electrons are not emittedalong a line because of the lack of emission in apertures 13 the smalldimensions of apertures i3 and the very limited area surrounding theapertures that is electron emissive produces a virtual line source whichfor practical purposes can be considered and treated as a line source.It is also possible to utilize substantially all of the electronsemitted from face t by the provision ofappropriate configurationthereof. In the illustrated embodiment wherein a line source is desired,racesis formed with a concave outer surface which serves to focus theemittedelectrons ina manner to be explained in more detailbelow.

For a practicalapplication of the improved electron source of thepresent invention, reference is made to Fig. 4 wherein there isillustrated a synchrotron injector mechanism including electron source Iin end view. There is provided in substantially enveloping relationshipto electron source l aeshicld lt with an elongated aperture lltherein'in alignment. with apertures l3 in face 4 and of a much greatersize than the apertures. At a distance from shield 18 and in substantialparallelism therewith is a second shield or electrade 18 in which thereis also provided an clongated aperture [9 of substantially the same sizeas aperture-v f'i in shield l6 and in alignment therewith.

Energization of the injector of Fig. 4 may be accomplished by theconnection of heater current supply 21 across conductors 8 of electronsource whereby portions of source face 4 are rendered electron emissive,as set forth above. Removal of the electrons emitted from. source isaccomplished by the establishment of an electrostatic acceleratingfieldhaving a polarity such that electrons are attracted from source t.This field is established by the application of a potential betweenshields iii and i8 by means of an acceler ating voltage supply 22connected between shields i6 and i8. Outer shield or electrode 18 iscon. nected to the positive terminal of accelerating voltage supply 22so that electrons emitted from face t of source l are attracted towardshield t8 and. pass through the aperture l9 therein i the form of abeam. In the illustrated embodiment of t e synchrotron injector, shieldis is electrioally connected to the envelope 6 of source I which therebyresults in the impression upon face 4 of a negativepotential fromaccelerating voltage supply 22. The concave surface of face is thusrelatively negatively charged and, accordingly, there is applied to theelectrons emitted from the vicinity of the center. line of thisconcavity on face 4 an electrostatic force tending .to repel theelectrons. As the same force is exerted above and below the line ofelectron emission the emitted electrons are influenced. to travel awayfrom face 63' in a directionperpendicular to the line of emission ratherthan at some other angle and the emergent electron beam is thus focusedinto a line perpendicular to the plane of Figure 4..

The above disclosure of the. invention has been made with reference tobut a single embodiment and. in. connection with only one application:thereof; however, it will be apparent that the numerous advantages ofthe invention may be realizedby the utilization of the variousmodifications possible within the scope of the invention. For example,numerous combinations of activating materials and source face materialsmay be employed; the operating temperature being varied as a result. Inthis respect, the operating temperature of barium aluminate as theactivating material with molybdenum is 1350 C., with zirconium is 1200'(3., and with titanium is 1050 C. It will be appreciated that thesetemperatures'need not be exactly attained and that electron emissionnaturally occurs over a range of temperatures; however, electronemission does decrease rather radically with temperature for substantialtemperature variations from those quoted above.

By virtue of the novel electron source construction the amount of heatloss in the source isminimized and further, in distinction from commonelectron sources of the type replacing the emission surface, electronemission occurs only at the designed location whereby no problems orlosses arise from extra or unwanted electron emission which cannot beefficiently utilized. It is also to be noted that construction of anelectron emissive surface in accordance with the present inventionmaterially intensifies and concentrates the beam of electrons availableat a distance from the source and thus'further increases the effectiveeificiency of the source. Under operating conditions an electron sourceconstructed according to the present invention has been found tocontinually emit ten times the number of electrons available from aconventional heated tungsten filament. The advantages of having anelectron emissive surface which is continually being replaced as in thepresent invention are multitudinous and include the lengthening of thelife of the surface and protection of the metal backing under normaloperation and the ability of the source to continue to emit normallyduring and after positive ion bombardment which would materially afiectelectron emission from conventional emission surfaces. Furthermore, theactivating material is protected from destructive ion bombardment.

In view of the numerous advantages of the invention and variety ofmodifications possible thereunder, the present invention is not to belimited by the necessarily brief and exemplary illustration of theinvention presented above, but instead the scope of the invention is tobe measured solely by the appended claims.

What is claimed is:

1. An electron source comprising in combination a metal wall having acylindrically concave surface and minute apertures formed through saidwall along the line of maximum concavity, an activating materialdisposed contiguous with said wall on the opposite side thereof fromsaid concavity and being readily vaporizable by heat, heating meansadjacent said activating material for vaporizing said material andheating said wall whereby vaporized activating material passes throughsaid apertures and coats the area directly adjacent thereto with asubstantially monoatomic layer of activating material to form anelectron emissive surface on said heated wall, and means establishingelectrostatic lines of force extending per pendicular to the concaveface of said metal wall whereby electrons emitted therefrom are focusedinto a substantial line parallel to said metal wall.

2. An electron source comprising a first box having an open side, asecond box having an open side and a metallic side opposite thereto,said second box slidably engaging said first box to form a closedenvelope, heating means disposed interior to said envelope, activatingmaterial disposed within said envelope about said heating means andtransmitting heat therefrom to said envelope, said activating materialbeing volatile upon the application of heat thereto, and the metallicside of said second box having minute apertures formed therein closelyadjacent and in a line of desired electron emission whereby vaporizedactivating material flows through said apertures and spreads in asubstantially monoatomic layer upon said metal wall about said aperturesto form in combination with said heated metal side an electron emissivesurface having a configuration determined by the disposition of saidapertures.

3. An electron source comprising an envelope having a metal wall,heating means internal to said envelope, and heating said envelope tosubstantially 1200 degrees centigrade, and powdered barium aluminatedisposed within said envelope about said heating means, said metal wallhaving apertures therein whereby barium aluminate vaporized by heat fromsaid heating means flows through said apertures to form a substantiallymonoatomic layer upon said metal wall, and said metal wall being formedof zirconium whereby copious quantities of electrons are emitted fromthe area directly adjacent said apertures and covered by said layer ofbarium aluminate.

4. An electron source as claimed in claim 3 wherein the heating unitheats the metal wall to substantially 1350 degrees centigrade and themetal face consists of molybdenum.

5. An electron source as claimed in claim 3 wherein the heating unitheats the metal wall to substantially 1050 degrees centigrade and themetal wall consists of titanium.

6. An electron source comprising a metallic envelope having acylindrically concave surface thereon and a plurality of aperturestherein along the line of maximum concavity, heating means interior tosaid envelope, an activating material about said heating means interiorto said envelope and being volatile at the temperature of said heatingmeans to slowly pass as a vapor through said apertures in the concavesurface of said envelope and form a thin layer upon the exterior of saidenvelope adjacent said apertures and thereby in combination with saidheated metal surface emit elec-- trons, an electrode positioned from theconcave surface of said envelope and having an aperture therein insubstantial alignment with said concave surface, and power supply meansimpressing a potential between said electrode and said envelope withsaid envelope being electrically negative relative to said electrodewhereby emitted electrons are focused into a beam and attracted awayfrom said envelope through the aperture in said electrode.

LESLIE J. COOK.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,929,931 Parker Oct. 10, 1933 2,107,945 Hull et al. Feb. 8,1938 2,131,204 Waldschmidt Sept. 27, 1938 2,416,661 Lawton Feb. 25, 1947

